1. Field of the Invention
The present invention relates to a vacuum type booster device for vehicles and particularly, to a vacuum type booster device capable of being highly responsive when a brake pedal is stepped on strongly.
2. Discussion of the Related Art
Generally, in vacuum type booster devices, when the stepping of a brake pedal advances an input rod which thus causes a plunger to advance relative to a valve piston, a vacuum valve comes into contact with a vacuum valve seat to block the communication between a variable pressure chamber and a constant pressure chamber. When the plunger is further advanced then, an atmosphere valve seat is separated from an atmosphere valve, whereby the atmospheric air is admitted from the surrounding atmosphere to the variable pressure chamber through a sound-absorbing member and a filter element. Thus, the valve piston is moved forward in dependence on a pressure difference between the variable pressure chamber and the constant pressure chamber to push a master piston, whereby a brake oil pressure corresponding to the steeping force of the brake pedal is generated in a master cylinder.
Since the valve piston elastically deforms a reaction member by an operation power corresponding to the pressure difference between the variable pressure chamber and the constant pressure chamber to push the master piston, the reaction member under the elastic deformation pushes the plunger rearward. This causes the plunger to retract, so that the atmosphere valve seat is seated on the atmosphere valve to block the communication of the variable pressure chamber with the atmosphere and hence, to hold the brake oil pressure at a desired pressure.
Although it is required to lead the atmospheric air to the variable pressure chamber without delay when the brake pedal is stepped on strongly, the use of the sound-absorbing member makes it difficult due to a ventilation resistance thereacross to lead a sufficient volume of the atmospheric air to the variable pressure chamber, and this makes a cause of difficulty in enhancing the responsiveness in braking operation. Particularly, improvement has been required these days with an increasing desire for shortening the time for vehicle to stop after braking.
As one satisfying such desire, there has been known a vacuum type booster device as described in U.S. Pat. No. 5,056,413 (equivalent of Japanese Examined Published Patent No. 6-24922). In the device described in the patent, a sleeve is provided around a valve housing (valve piston), and an additional annular air passage is defined between the sleeve and the valve housing to open to a rearward chamber (variable pressure chamber). An additional valve member is formed at the rear end of the sleeve and is biased toward a third valve seat so that the same is usually held closed by the additional valve member. When a brake pedal is stepped on sharply to advance an operating member (input rod) beyond a predetermined distance relative to the valve housing, the additional valve member formed at the rear end of the sleeve is separated from the third valve seat. This causes the atmospheric air from a filter element not only to flow into the rearward chamber through an inner space and an intermediate space, but also to flow into the rearward chamber through the additional air passage. As a consequence, the pressure in the rearward chamber rapidly comes to be equal to the atmospheric air, so that the braking responsiveness can be enhanced.
However, in the device described in the aforementioned patent, since an additional sleeve defining the additional air passage is provided around the valve housing, a problem arises in that the vacuum type booster device is enlarged in dimension. Further, since the third valve seat has to be newly provided for communication/blocking of the additional air passage with the atmosphere, there arises another problem that the reliability as product is lowered. In addition, the valve housing and the sleeve are required to be slidable, wherein sliding support portions therefor receive all the loads which act on the valve housing, and are very difficult to perform smooth sliding, so that an additional problem arises in that the functions in the beginning are difficult to be performed stably.